Ultrasonic detection systems are well known for the protection of buildings, warehouses and other areas to provide an alarm signal or other notice when an intruder or other unauthorized object passes within the area protected by the system. In applicant's application, Ser. No. 455,260, a particular form of such system is disclosed which provides signal detection and noise immunity and other features which contribute to the overall reliability of detecting unauthorized entry into the protected zone. These prior systems can be arranged with multiple transducers to protect a larger area and can be duplicated to provide protection in adjacent or related areas. The cost of installation, calibratio, periodic checking of system sensitivity and routine maintenancce all, of course, increase in direct relation to the number of such systems which are installed.